Viscoelastic behaviour and time-temperature correspondence of HDPE with varying levels of process-induced orientation

Sheer controlled orientation in injection moulding (SCORIM) is a non-conven tional injection moulding technique that allows for the enhancement of the mechanical properties of semi-crystalline polymers. In this work the flexur al mechanical feature, namely their time-dependence, of conventionally inje ction-moulded high-density polyethylene (HDPE) (PEc) and SCORIM-processed H DPE (PEs) are compared. The differences should be attributed to the appeara nce of highly oriented structures such as shish-kebab morphologies in the l atter case. The quasi-static performance of HDPE shows a 59% increase in th e flexural modulus following SCORIM application. Master curves of the frequ ency-dependence of the storage modulus and of the time-dependence of strain were constructed successfully in the region of the ol-relaxation. The visc oelastic data are well described by the Cole-Cole model. The wide broadness of the distribution of relaxation time observed for both materials is ascr ibed to the molecular mobility of the amorphous phase involved in the relax ation process. Two approaches were used to investigate the curvature observ ed in the Arrhenius plots. In the first case, two separate processes were a ssumed to contribute to the a-relaxation, with activation energies of 150-1 70 and 180-210 kJ mol(-1), respectively (dynamic data). However, the two al pha -processes, alphaI and alpha II, often reported in the literature, did not appear explicitly in the original data, which showed a single peak in a n E " vs T plot. Therefore, a unique process for the a-relaxation was also assumed and was well described taking into account changes in the reduced v olume whose temperature-dependence obeys a general equation of state. This concept enabled a good agreement between creep and dynamic experiments. (C) 2001 Elsevier Science Ltd. All rights reserved.